This invention relates to an information recording and reproducing system using a recording medium which is capable of recording and reproducing an information signal such as an optical disk, and more particularly, it relates to an information recording and reproducing system which enables a secure tracking at the time of recording and reproducing the signal.
Referring to FIG. 1A, the referential numeral 1 represents a drive circuit of a semiconductor laser 2. FIG. 1B is a graph explaining the principle of operation of the semiconductor laser. The semiconductor laser 2 can modulate the output level (the ordinate in FIG. 1B) of laser light quantity directly from y.sub.1 to y.sub.2 by the pulse modulation from x.sub.1 to x.sub.2 of the level of drive current (the abscissa in FIG. 1B) which corresponds to a reproduction drive signal 3 or a record drive signal 4 which is introduced to the drive circuit 1. At the time of reproduction, the laser 2 is operated with a laser light quantity output y.sub.1 and, at the time of recording with a laser light quantity output y.sub.2.
A method for recording information will be first described. The light modulated by an information signal as described above and emitted from the laser 2 becomes a beam 11 which has been caused to converge by a coupling lens 5, a polarizing beam splitter 6, a galvanomirror 7, a quarter wave plate 8 and an objective lens 9, and on a disk 10, which is irradiated by the laser beam, an information signal consisting of pits is recorded.
A method for reproducing the information signal recorded in this way will next be explained. When the laser beam, the output level of laser light quantity of which has been decreased as described above, is caused to irradiate pits (not shown) which are formed on a track 12, the pits diffract the beam and the diffracted reflected beam returns to a photodetector 13 through the above optical system. At this time, due to the polarization effect of the quarter wave plate 8, it is reflected by the polarizing beam splitter 6, detected by the photodetector 13 and converted to an electrical signal so that it may be taken out as an output 13'.
FIG. 2 shows an example of the structure of a track which has hitherto been used for such an optical disk apparatus capable of recording and reproducing. In the track 12, a header area 121 and an information recording area 122 are disposed alternately, and in the header area 121 a track address, a sector address, a synchronization signal and the like are recorded in advance with the pits .lambda./4 deep. In the header area 121 and the information recording area 122 a track groove .lambda./8 deep is formed in advance and the data pits are recorded on this track groove. If a light beam deviates from the center of the groove of the track 12, the diffracted beam pattern on the photo detector in FIG. 1A becomes asymmetric by virtue of the edges of the track groove. Therefore, in this kind of optical disk apparatus, as is shown in FIG. 8, the diffracted beam from a track groove is received by two photodetectors 131, 132 arranged in parallel with the track groove, and a tracking signal is detected by differentiating the outputs of these two detectors. This kind of apparatus has been disclosed in the Japanese patent application Laid-Open No. 60702/74.
In the .lambda./8 deep track type tracking method described above, however, when a light beam is moved by, for example, a galvanomirror for conducting tracking control, a tracking signal offset is produced on a tracking signal. Further, when the disk is not tilted, the comma aberration is produced, which disturbs, particularly, the balance of the two photodetectors which are arranged in parallel with the track and increases the probability of a tracking signal offset, thus disadvantageously making normal tracking impossible.
In addition, when the information recorded with high density in a DC groove is reproduced, tracking sensitivity is lowered, which unfavorably increases the tracking offset. Furthermore, in the conventional .lambda./8 deep DC groove tracking method, the tracking signal inconveniently lacks stability because of the difference of reflectivity, evenness and refractive index of the disk which are inevitable due to the properties of the recording materials, and therefore, there are limitations in the degree of high-density recording possible.